913 research outputs found
On Electron Transport in ZrB12, ZrB2 and MgB2
We report on measurements of the temperature dependence of resistivity,
, for single crystal samples of ZrB, ZrB and
polycrystalline samples of MgB. It is shown that cluster compound
ZrB behaves like a simple metal in the normal state, with a typical
Bloch -- Gr\"uneisen dependence. However, the resistive Debye
temperature, , is three times smaller than obtained from
specific heat data. We observe the term in of these borides,
which could be interpreted as an indication of strong electron-electron
interaction. Although the dependence of ZrB reveals a sharp
superconductive transition at , no superconductivity was observed
for single crystal samples of ZrB down to .Comment: 5 pages, 4 figure
Full Relativistic Electronic Structure and Fermi Surface Sheets of the First Honeycomb-Lattice Pnictide Superconductor SrPtAs
We report full-potential density functional theory (DFT)-based {\it ab
initio} band structure calculations to investigate electronic structure
properties of the first pnictide superconductor with a honeycomb-lattice
structure: SrPtAs. As a result, electronic bands, density of states, Fermi
velocities and the topology of the Fermi surface for SrPtAs are obtained. These
quantities are discussed in comparison to the first available experimental
data. Predictions for future measurements are provided
Electronic Structure of New AFFeAs Prototype of Iron Arsenide Superconductors
This work is provoked by recent discovery of new class prototype systems
AFFeAs (A=Sr,Ca) of novel layered ironpnictide High-Tc superconductors
(Tc=36K). Here we report ab initio LDA results for electronic structure of the
AFFeAs systems. We provide detailed comparison between electronic properties of
both new systems and reference LaOFeAs (La111) compound. In the vicinity of the
Fermi level all three systems have essentially the same band dispersions.
However for iron fluoride systems F(2p) states were found to be separated in
energy from As(4p) ones in contrast to La111, where O(2p) states strongly
overlaps with As(4p). Thus it should be more plausible to include only Fe(3d)
and As(4p) orbitals into a realistic noninteracting model than for La111.
Moreover Sr substitution with smaller ionic radius Ca in AFFeAs materials leads
to a lattice contruction and stronger Fe(3d)-As(4p) hybridization resulting in
smaller value of the density of states at the Fermi level in the case of Ca
compound. So to some extend Ca system reminds RE111 with later Rare Earths.
However Fermi surface of new fluorides is found to be nearly perfect
two-dimensional. Also we do not expect strong dependence of superconducting
properties with respect to different types of A substitutes.Comment: 5 pages, 4 figure
Contrasting Pressure Effects in Sr2VFeAsO3 and Sr2ScFePO3
We report the resistivity measurements under pressure of two Fe-based
superconductors with a thick perovskite oxide layer, Sr2VFeAsO3 and Sr2ScFePO3.
The superconducting transition temperature Tc of Sr2VFeAsO3 markedly increases
with increasing pressure. Its onset value, which was Tc{onset}=36.4 K at
ambient pressure, increases to Tc{onset}=46.0 K at ~4 GPa, ensuring the
potential of the "21113" system as a high-Tc material. However, the
superconductivity of Sr2ScFePO3 is strongly suppressed under pressure. The
Tc{onset} of ~16 K decreases to ~5 K at ~4 GPa, and the zero-resistance state
is almost lost. We discuss the factor that induces this contrasting pressure
effect.Comment: 5 pages, 4 figures, to be published in J. Phys. Soc. Jpn. No.12
(2009
Magnetic Ordering in V-Layers of the Superconducting System of Sr2VFeAsO3
Results of transport, magnetic, thermal, and 75As-NMR measurements are
presented for superconducting Sr2VFeAsO3 with an alternating stack of FeAs and
perovskite-like block layers. Although apparent anomalies in magnetic and
thermal properties have been observed at ~150 K, no anomaly in transport
behaviors has been observed at around the same temperature. These results
indicate that V ions in the Sr2VO3-block layers have localized magnetic moments
and that V-electrons do not contribute to the Fermi surface. The electronic
characteristics of Sr2VFeAsO3 are considered to be common to those of other
superconducting systems with Fe-pnictogen layers.Comment: 4 pages, 4 figures, To appear in JPSJ 79 (2010) 12371
Electron transport, penetration depth and upper critical magnetic field of ZrB12 and MgB2
We report on the synthesis and measurements of the temperature dependence of
resistivity, R(T), the penetration depth, l(T), and upper critical magnetic
field, Hc2(T), for polycrystalline samples of dodecaboride ZrB12 and diboride
MgB2. We conclude that ZrB12 as well as MgB2 behave like simple metals in the
normal state with usual Bloch-Gruneisen temperature dependence of resistivity
and with rather low resistive Debye temperature, TR=280 K, for ZrB12 (as
compared to MgB2 with TR=900 K). The R(T) and l(T) dependencies of ZrB12 reveal
a superconducting transition at Tc=6.0 K. Although a clear exponential
l(T)dependence in MgB2 thin films and ceramic pellets was observed at low
temperatures, this dependence was almost linear for ZrB12 below Tc/2. These
features indicate s-wave pairing state in MgB2, whereas a d-wave pairing state
is possible in ZrB12. A fit to the data gives a reduced energy gap
2D(0)/kTc=1.6 for MgB2 films and pellets, in good agreement with published data
for 3D \pi - sheets of the Fermi surface. Contrary to conventional theories we
found a linear temperature dependence of Hc2(T) for ZrB12 (Hc2(0)=0.15 T).Comment: 8 pages, 10 figures, submitted to JET
Soil water retention curve of agrogray soils: Influence of anisotropy and the scaling factor
Contrast of LiFeAs with isostructural, isoelectronic, and non-superconducting MgFeGe
Stoichiometric LiFeAs at ambient pressure is an 18 K superconductor while
isoelectronic MgFeGe is not, despite their extremely similar electronic
structures. To investigate possible sources of this distinctively different
superconducting behavior, we quantify the differences using first principles
density functional theory, establishing first that the Fe total 3d occupations
are identical in the two compounds. Individual 3d orbital occupations also
differ very little (). The differences in Fermi surfaces (FSs) do
not seem significant; however a redistribution of bands just above the Fermi
level does represent a possibly significant distinction. Because the bands and
FSs of LiFeAs are less in agreement with experiment than for other
iron-pnictides, we study the effects of additional exchange-correlations
effects beyond GGA (the generalized gradient approximation) by applying the
modified Becke-Johnson potential (mBJ) exchange potential, which gives much
improved bandgaps in insulators compared to GGA and might be useful for
semimetals such as the Fe-based superconductors. Overall, we conclude that the
mBJ corrections do not improve the description of LiFeAs as compared to
experiment
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